Neuroanatomical substrates of circuit specific cholinergic modulation across the primate anterior cingulate cortex.

Abstract

Acetylcholine is a robust neuromodulator of the limbic system and a critical regulator of arousal, emotions, and stress. The anterior cingulate cortex (ACC) and the amygdala (AMY) are key limbic structures that are both densely innervated by cholinergic afferents and interact with each other for emotional regulation. The ACC is comprised of functionally-distinct dorsal (A24), rostral (A32), and ventral (A25) areas that differ in their connections with the AMY. The structural substrates of cholinergic modulation of distinct ACC microcircuits and outputs to AMY are thought to depend on the laminar and subcellular localization of cholinergic receptors. The present study examines the distribution of muscarinic acetylcholine receptors, m1 and m2, on distinct excitatory and inhibitory neuronal classes and on AMY-targeting projection neurons within ACC areas, via immunohistochemistry and in vivo injections of neural tracers into the basolateral AMY in adult rhesus monkeys of both sexes. We found that laminar densities of m1+ and m2+ expressing excitatory and inhibitory neurons depended on area and cell-type. Among the ACC areas, ventral subgenual ACC A25 exhibited greater m2+ localization on presynaptic inhibitory axon terminals and greater density of m1+ and m2+ expressing AMY-targeting (tracer+) pyramidal neurons. These patterns suggest robust cholinergic dis-inhibition and potentiation of amygdalar outputs from the limbic ventral ACC, which may be linked to the hyperexcitability of this subgenual ACC area in depression. These findings reveal the anatomical substrate of diverse cholinergic modulation of specific ACC microcircuits and amygdalar outputs that mediate cognitive-emotional integration and dysfunctions underlying stress and affective disorders.Significance Statement Cholinergic neuromodulation of anterior cingulate (ACC) cortico-limbic networks are consequential for cognitive-emotional integration. This study explored muscarinic acetylcholine receptor (m1 and m2) expression on distinct excitatory and inhibitory neurons across functionally-distinct ACC subregions and their outputs to the amygdala, a key limbic structure for emotional regulation. Our findings reveal that muscarinic receptor expression is most robust in ventral subgenual ACC area 25, in patterns that suggest cholinergic dis-inhibition and potentiation of area 25 outputs to the amygdala. These neuromodulatory interactions may be linked to the hyperexcitability of subgenual ACC found in depression. Our findings yield novel insight on how cholinergic modulation of specific ACC microcircuits and amygdalar outputs can contribute to cognitive-emotional processing and dysfunction in stress and affective disorders.